Quadrature signals are common in communication systems and chips for use in communication systems. In particular, there are many needs for a quadrature signal generator that operates over a wide range of frequencies, including the need for a local oscillator (LO) signal in digital transmitters for multi-mode, multi-band communication devices, such as cellular telephones or radios. As complexity rises and performance demands increase, signal-to-noise and power issues often result. Generation of quadrature signals with desired signal-to-noise ratios has proven difficult.
Communications signals are often implanted on a carrier signal and modulated. Numerous modulated carrier signals may be simultaneously transmitted as long as the signals are transmitted upon differing radio frequency channels of the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands and have regulated transmission of the modulated carrier signals upon various ones of the frequency bands. It should be noted that frequency bands are further divided into channels, and such channels form the radio-frequency channels of a radio communication system.
Quadrature amplitude modulation (QAM) is a modulation technique which may be advantageously utilized to transmit efficiently a communication signal encoded into discrete form. More particularly, one particular QAM modulation technique is a Π/4-differential quadrature phase shift keying (or Π/4-DQPSK) modulation technique. Such modulation technique has been selected as a modulation standard for several cellular communication systems. In a Π/4-DQPSK modulation technique, the binary data stream into which the communication signal is encoded is separated into bit pairs. Such bit pairs are utilized to cause phase shifts of a carrier wave in increments of plus or minus Π/4 radians or plus or minus 3 Π/4 radians according to the values of individual bit pairs of the encoded signal. Such phase shifts are effectuated by applying the binary data stream comprised of the bit pairs to a pair of mixer circuits. A sine component of a carrier signal is applied to an input of a first mixer circuit of the pair of mixer circuits, and a cosine component of the carrier signal is applied to an input of a second mixer circuit of the pair of mixer circuits. It should be noted that the sine and cosine components of the carrier signal are in a relative phase relationship of ninety degrees with one another. A quadrature signal generator is utilized to apply the sine and cosine components of the carrier signal to the first and second mixer circuits of the pair of mixer circuits, respectively.
A quadrature signal generator may be formed of a resistor-capacitor pair in which the value of at least either the resistor or the capacitor is variable as a function of voltage. The relative phase of the signals generated by a quadrature signal generator are dependent upon the values of the resistor-capacitor pair, and, as the values of the resistor and capacitor of the resistor-capacitor pair are functions of voltage, the range of frequencies over which quadrature can be generated by the quadrature signal generator is dependent upon voltage levels of phase-controlling voltages applied to the quadrature signal generator.
As the circuitry of apparatus, such as a radiotelephone utilized in a cellular, communication system, of which the quadrature signal generators form a portion, are constructed to be operated at ever-lower voltage levels, the range of values of which the resistor or capacitor of the resistor-capacitor pair can take is increasingly limited. The range of frequencies of signals generated by a quadrature signal generator so constructed is increasingly limited.
A quadrature signal generator may alternately be constructed of a flip-flop pair arranged such that the outputs of each flip-flop of the flip-flop pair are applied to inputs of the other flip-flop of the flip-flop pair. A clock signal is also applied to each of the flip-flops of the flip-flop pair wherein the clock signal is inverted prior to application to one of the flip-flops. Outputs of the respective flip-flops of the flip-flop pair are in a ninety degree phase relationship (and, hence, are in phase quadrature) when the duty cycle of the clock signal applied to the flip-flops is of a fifty percent (50%) duty cycle. That is, the clock signal must be of a high logic level for exactly half of the period of the clock signal and be of a low logic level for exactly half of the period of the clock signal.
Any variation in the duty cycle of the clock signal causes the signal output by the respective ones of the flip-flop pair to be out-of-phase quadrature (i.e., in a phase relationship other than a ninety degree phase relationship) with one another. When the duty cycle of the clock signal is significantly different than a fifty percent (50%) duty cycle, the signals generated by the flip-flop pair are significantly out-of-phase quadrature.
Clock oscillators which generate clock signals will not in general produce clock signals exactly of the fifty percent (50%) duty cycle. Additionally, the duty cycle of the clock signal generated by a clock oscillator may vary as the clock oscillator ages or as a result of circuit placement of the clock oscillator.
Prior art attempts to generate quadrature signals have included the use of frequency doublers, but such signal processing generally results in an output half the signal strength of the input. This result may be acceptable, in certain applications, but adverse effects on signal-to-noise performance can render this approach problematic.